An optical fiber communication technology has made substantial progress in increasing the capacity and the distance by means of high-speed intensity modulation techniques and wavelength multiplexing techniques. In addition to this, the improvement of digital signal processing technologies has enabled to utilize polarization multiplexing technologies and multi-level phase modulation technologies in recent years. This makes it possible to dramatically increase the transmission capacity using existing optical fiber networks.
The multi-level phase modulation technology requires in a receiver an element called an optical hybrid mixer to convert phase modulation optical signals into intensity modulation optical signals. The optical hybrid mixer converts a phase variation of the phase-modulated signal light into the intensity modulation optical signal by making signal light having propagated through an optical fiber interfere with local oscillation light (local light) having a constant amplitude and phase without modulation. Such optical hybrid mixer requires an optical waveguide with excellent optical characteristics.
On the other hand, the downsizing and cost reduction of the optical hybrid mixer have strongly been required to transmit a larger amount of signals over a longer distance at a lower cost. Since a silicon optical waveguide strongly confines the light within an optical waveguide, it makes possible to dramatically reduce the size compared with a glass-based optical waveguide that has been mainly used in the past. Many technologies have been developed to realize the silicon optical waveguide utilizing CMOS (complementary metal oxide semiconductor) process technologies used for manufacturing LSI (large scale integration) circuits (see WO 2014-125535, for example).
Because the silicon optical waveguide is made of silicon material whose refractive index is large, a propagation mode size is very small compared with that of a glass waveguide made mainly of silica glass. This makes the propagation mode vary sensitively according to the variation of a core shape. Accordingly, extreme processing accuracy is required in comparison with that in a case in which the optical waveguide is formed by glass processing technologies. For example, the core width of the glass waveguide is controlled with plus or minus 0.1 micrometers (μm) accuracy, but the core width of the silicon waveguide needs controlling with plus or minus several nanometers (nm) accuracy.
It is expected to apply the CMOS process technology, which can realize such high processing accuracy, to the optical waveguide technology.
In an electronic device, it is required to control the width of a local pattern such as a gate electrode of a MOS transistor or the like. In contrast, in the optical waveguide, in order to control a phase stably, it is necessary to stably control a continuous pattern in length from several tens of micrometers (nm) to several hundred micrometers (μm), or in millimeter order length in some cases. Some sort of trouble can occur that does not affect the electronic device. Thus, there is a technical problem that needs to be solved to realize the silicon optical waveguide with intended characteristics by using the CMOS process technology.
On the other hand, aside from the above-mentioned processing technology, an example of technologies is described in WO 2011-115285 by which the difference in optical path length of the optical waveguide is correctly controlled. The optical waveguide described in WO 2011-115285 is a pair of optical waveguides having a curved section, and this curved section is composed of at least an arc-like waveguide section having the same curvature. The pair of optical waveguides is configured in which the number of the arc-like waveguide sections included in each optical waveguide is equal to each other. It is said that such configuration enables the difference in optical path length of a pair of optical waveguides to be correctly controlled in a pair of optical waveguides each of which is composed of a combination of a lot of curved waveguide sections.